Gear pump for metering and extruding hot organic thermoplastics



July 18, 1950 F. E. DULMAGE ETAL 2,515,201

GEAR PUMP FOR METERING AND EXTRUDING HOT ORGANIC THERMOPLASTICS Filed May 27, 1948 2 Sheets-Sheet 1 11V VEN TORS. Frederic/4 E. Du/m age ATTORNEYS Gran/ W Cheney July 18, 1950 F. E. DULMA ETAL 2,515,201

7 GEAR PUMP F 0R TERI TRUDING HOT ORGA THERMOP CS Filed May 27, 1948 2 Sheets-Sheet 2 21 (T 11. I I G 17 H n; QIIWH HHW HHHI HW 17 I E 25 15 E 1 w i Q (Q) l 1N VEN TORS. Frederick E. Du/mqge Gram W Cheney ATTORNEXS Patented July 18, 1950 GEAR PUMP FOR METERING AND EXTRUD- ING HOT ORGANIC THERMOPLASTICS Frederick E. Dulmage, Saginaw, and Grant W. Cheney, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application May 27, 1948, Serial No. 29,514

This invention relates to a non-pulsating gear pump for hot organic thermoplastics. It relates specifically to such a pump capable of delivering the thermoplastic at a constant rate in a sheet free from strain and of uniform width and thickness.

According to the invention, such a pump comprises a heat jacketed pump case with two coacting toothed rotors mounted therein, having a particular arrangement and type of teeth thereon, an inlet for molten thermoplastic and a long narrow discharge orifice, all to be described more fully hereinafter.

The invention will be illustrated by and described with reference to the accompanying drawings, wherein Fig. 1 is a vertical cross-section through the pump case without the rotor members in place;

Fig. 2 is a perspective view of the rotors, separated from the pump case, showing one preferred arrangement of the gear teeth;

Fig. 3 is a fragmentary view of the rotors, showing another preferred arrangement of the teeth;

Fig. 4 is an external view of the inlet side of the pump showing, as well, the drive shaft for the rotors.

The stationary housing l or case of the new pump has a single cavity II for mating rotors l2 (to be described later), with an inlet l3 for the hot plastic centered in one wall of the cavity ll facing the nip between the rotors l2 and an adjustable orifice I4, as long as the rotors I2, to receive the polymer discharged from the said rotors l2 in a plane passing through said nip and said inlet l3. The term nip when used herein is intended to signify the horizontal line of contact along which the rotors l2 mate and part during their rotation, in analogy to the more common use of that term to describe the line of contact between a pair of smooth cylindrical pinch rolls. The rotor cavity II is in the form of two longitudinally intersecting equal right cylinders whose axes are parallel to one another but spaced apart a distance which is less than their diameters and is equal to the pitch diameter of the equal rotors I2 intended to operate therein. On the inlet side of the rotor cavity ll, extending longitudinally each way from the centrally disposed inlet port I! to the flat end plates I5, is a distributor groove or channel l6 along which the molten plastic feed may be distributed under pressure to face the entire length of each of the rotors l2 operating in said cavity l I.

The rotary members l2 are each composed of a rotor shaft l1 upon which has been mounted a ll 3 Claims- (Cl. 103-128) plurality of identical multiple-tooth gear wheels l8, each havin a pitch diameter equal to the distance between centers of the rotor shafts l'l. These wheels I8 are secured to the rotor shaft and to one another (with no intervening space along the shaft) in such a manner that each tooth IS on one wheel 3 is offset from those of adjacent wheels l8 on the same shaft I! by an amount of 0.15 to 0.4, and preferably about .33 times, the circular pitch of the teeth. Thus, when each wheel has 20 teeth, each is offset uniformly by a value of from about 2.7 to about 72 and preferably about 6 from the corresponding tooth I 9 of each of the adjacent wheels l8 on the same shaft ll.

The offset pattern of teeth I9 along the shaft I! may be continuously of the same hand, so that the rotor faces present the appearance of having a plurality of helical flights, as illustrated in Fig. 2, but it is found preferable to arrange the wheels IS with corresponding teeth [9 offset in a repetitive pattern having a resemblance to herringbone gears, as illustrated in Fig. 3. Thus, in the example given above, with each wheel offset 6 from its neighbors, the offset pattern is 0, 6, 12, 6, 0, 6, 12, It is to be understood that, despite reference above to gear patterns resembling helical flights and those resembling herringbone gears, there is no helical or zig-zag movement of plastic during operation of the pump, since each flute 20 between the teeth IS on each gear wheel I8 is filled with plastic in passing the supply header or groove l6, and there is no thrust along the pump on any of the matter being pumped.

The shape of the teeth on the rotors is important to successful operation of the pump. Each tooth has a smoothly rounded or semicircular crown, when viewed in elevation, and each interdental flute is an exact fit for the teeth, having a smoothly rounded or semicircular contour of the same radius as that of the teeth. The teeth are not significantly undercut, so that, in mating position, each tooth completely fills its companion flute without clearance and there is no possibility of any of the hot plastic remaining in the flutes for a second trip around the pump. Instead, overheatin and burning are eliminated because the plastic makes a single linear pass through the equipment.

The teeth I! on one rotor I! are positioned as the mirror image of those on the other rotor l2 so'that, in operation, the rotors I! will mate perfectly at the nip between them when driven in opposite directions. Extensions of the rotor shafts l1, through the end plates I of the pump case I0 carry companion gears 2I and are driven, by means not shown, so that the pump teeth [9 part on the inlet side of the pump, slide along the adjacent closely fitting wall of cavity II toward the outlet side of the pump, and cleanse themselves of plastic due to their perfect fit as they mate on the outlet side of the nip.

The inlet port I3 is adapted to be secured to.

and fed by the outlet of a continuous, screw-type plastics extruder (not shown) or by any suitable supply of molten plastic under constant pressure.

Because of the close fit between the cavity II of pump case I0 and the teeth IQ of the contained rotors I2, and the danger of irreparable damage,

due to any solid particles which might be carried into the pump, the inlet port I3 is guarded by a filter screen 22 supported by a perforate having communication through a small relief port 25 extending from the distributor groove I6 to the outside of the pump case l0, suitably through one end plate I5. In operation, the resection may be produced by suitable flaring oi fishtailing," or other variation in the shape of the orifice.

As shown in the drawings, the pump case l0 contains a jacket 23 for circulation of a heating fiuid around the inlet port I3, the rotor cavity II and the discharge passage 26, to assist in keeping the plastic being pumped at a constant temperature until it emergesthrough the orifice I4. It is preferred, as well, to heat the jaws 21 of the orifice I4, as by electrical resistance heaters (not shown) which may be installed in sockets 29 bored in said jaws 21. The rotors I2 may be heated, too, by installing electric heaters (not shown) in the rotor shafts ll, connection being made in known manner to an electric source during rotation of these shafts, through brushes slidable on commutators mounted in the end of the shafts.

The bearings 30 for the rotors I2, located in the end plates I5 of the pump case III. are lubricated by the plastic being pumped and, since that plastic is supplied under positive pressure,'there 1 is no contamination of the main body of plastic lief valve 24 is adjusted to permit a continuous discharge of a small stream of hot plastic through the relief port 25 to show that adequate pressure is being maintained. An equivalent adjustment is possible, without changing, the relief valve setting, by increasing or decreasing the rate of feed of plastic to the pump rotors I2 to maintain a continuous but minimum stream of plastic bleeding from the relief port 25. Y

The discharge orifice Ilcommunicates with and receives plastic from the discharge side of the rotor cavity II through aheated horizontal passageway 26 having asmall vertical dimension but being of the same width-as the length of the working faces of the pump rotors. I2. The orifice I4 is of adjustable thickness, being provided with movable jaws 21 whereby the extruded sheet of constant full width may be made thinner or thicker as desired up to the vertical dimension of said heated horizontal passageway 26. There is ordinarily no obstruction or other restricting or directing bathe in the outlet passage 26 or the orifice It, so that no particle of the hot plastic being discharged is diverted from its straight line path from the time it enters the flutes 20 of one of the rotors I2 until it has become immobilized in the cooled and hardened extruded sheet. The advantages of. this arrangement are apparent when the extruded sheet is examined under polarized light. It is found to be free from strain and, when made of colored plastic, to be free from striations. By way of contrast, sheets of the same plastics and of the same dimensions, produced by extrusion from a screw extruder directly through a slit orifice invariably show lines of strain and often have visible color striations due to "memory" of variations in the thermal and mechanical treatment of the plastic in various parts of the sheet. 1

While the preferred form of orifice is the linear slit illustrated in the drawings, it is to be understood that the successful operation of the pump is not dependent upon the use of such an orifice. Extruded strips of other than rectangular crosspassing through the pump by any foreign matter from the bearings 30. 1

In a specific embodiment, a pump was constructed as illustrated in the drawings. The intersecting cylinders forming the rotor cavity II each had a diameter of 3.595 inches, and'their tributor groove I6 was a segment of a cylinder of 1' inch radius whose longitudinal axis was the line of intersection of the two larger cylinders constituting the rotor cavity II. The horizontal outlet passage 26 was 6 inches wide and 0.375 inch deep, and extended to an adjustable 6-inch orifice 4.5 inches from the vertical center line of the rotor cavity II. Each rotor. shaft I! was about 1.7 inches in diameter and had mounted thereon 12 gear wheels 0.5 inch thick, having an outside diameter of 3.595 inches, a root diameter of 3.071 inches and a pitch diameter of 3.333 inches. Each gear member I8 had 20 teeth with a radius of 0.131 inch, a chordal thickness of 0.2615 inch, a circular thickness of 0.2618 inch, and a circular pitch of 0.5236 inch, and 20 interdental flutes of exactly the same dimensions and configuration. Adjacent gears I8 were mounted on their common shaft l1 and secured thereto with their teeth I9 staggered or offset from those of the neighboring gears I! by increments of 6", following the pseudo-herringbone pattern previously described. The rotors I2 were driven at speeds from 0.5 to 5 revolutions per minute, turning in opposite directions with their teeth I9 sliding along the wall of the rotor cavity I I from the inlet toward theoutlet side, and delivering hot plastic to the outlet passage 26 at the rate of about 0.5 pound per revolution when operating on polystyrene or ethyl cellulose and at slightly different rates, varying with the density.

when operating on other thermoplastics. In opwere heated internally by electrical resistance heaters.

Because of the close tolerance between the pump cavity H and its contained rotors ii,

there is aconstant volume of thermoplastic disit is merely necessary to use any of several known means for counting the revolutions of the geared rotors I2. I

The apparatus has been designated as a pump, but this should not be construed in the narrow sense of an apparatus for hastening the flow of the hot thermoplastic from the extruder to the orifice. Rather, it is an apparatus for forwarding the plastic at a steady rate (which may be quite slow), free from the pulsations common in extruders and in many pumps, from a zone of high pressure to the atmospheric pressure outside the orifice l4, and for distributing the plastic across the orifice in a form free from internal strain and striations.

We claim:

1. A gear pump adapted to receive a molten organic thermoplastic under pressure and to discharge it as a continuous sheet, comprising: a stationary case having a cavity therein for mating rotors, said cavity being in the form of two longitudinally intersecting right cylinders Whose axes are parallel and spaced apart in a vertical plane a distance equal to the pitch diameter of the rotors operative therein; fiat end plates closing said rotor cavity and having bearings therein for a rotor centered in each of said cylinders constituting said cavity; two rotary members extending between the said bearings in the end plates, through said cavity, each comprising a rotor shaft on which is mounted and secured a plurality of identical multiple tooth gear wheels filling the shaft between the end plates with no intervening space along the shaft, each gear wheel having an outside diameter substantially identical with the diameter of said cylinders to provide a sliding fit therein and a pitch diameter equal to the distance between the said parallel axes of the cylindrical portions of the rotor cavity, each tooth on each wheelbeing offset from those on adjacent wheels on the same shaft by from 0.15 to 0.4 times the circular pitch of the teeth; each two adjacent teeth on each wheel having a flute therebetween; each tooth, in elevation, exhibiting a smoothly rounded semicircular crown with no significant undercut and each interdental flute being smoothly semicircular of the same radius as the crown to provide an exact fit for its mating tooth; the teeth on one rotor being positioned as the mirror image complements of the flutes on the other rotor to provide perfect mating between corresponding wheels on the two shafts; an inlet port through and centered in one wall of the case facing the parting line between the rotors; an internal distributor groove on the inlet side of the pump parallel to and facing the parting line between the rotors extending throughout their length; an orifice extending between the end plates in communication with a horizontal passageway having a uniform small vertical dimension greater than that of the orifice and having the same length as the working face of the rotors, said passageway being in communication with the rotor cavity in the horizontal plane through the mating line between the rotors and on the opposite side of the pump from the said inlet port; means for heating the case, rotors and orifice; and means for driving the rotors at the same speed and in opposite directions such that the gear teeth on the internal rotors slide along the wall of the rotor cavity from the inlet toward the outlet side of the pump.

2. A gear pump adapted to receive a molten organic thermoplastic under pressure and to discharge it as a continuous sheet, comprising: a stationary case having a cavity therein for mating rotors, said cavity being in the form of two longitudinally intersecting right cylinders whose axes are parallel and spaced apart in a vertical plane a distance equal to the pitch diameter-of the rotors operative therein; flat end plates closing said rotor cavity and having bearings therein for a rotor centered in each of said cylinders constituting said cavity; two rotary members extending between the said bearings in the end plates, through said cavity, each comprising a rotor shaft on which is mounted and secured a plurality of identical multiple tooth gear wheels filling the shaft between the end plates with no intervenin space along the shaft, each gear wheel having an outside diameter substantially identical with the diameter of said cylinders to provide a sliding fit therein and a pitch diameter equal to the distance between the said parallel axes of the cylindrical portions of the rotor cavity, each tooth on each wheel being offset from those on adjacent wheels on the same shaft by about 0.33 times the circular pitch of the teeth; each two adjacent teeth on each wheel having a, flute therebetween; the teeth on one rotor being positioned as the mirror image complements of the flutes on the other rotor to provide perfect mating between corresponding wheels on the two shafts; an inlet port through and centered in one wall of the case facing the parting line between the rotors; an internal distributor groove on the inlet side of the pump parallel to and facing the parting line between the rotors and extending throughout their length; an orifice extending between the end plates in communication with a horizontal passageway having a uniform small vertical dimension greater than that of the orifice and having the same length as the working face of the rotors, said passageway being in communication with the rotor cavity in the horizontal plane through the mating line between the rotors and on the opposite side of the pump from the said inlet port; means for heating the case, rotors and orifice; and means for driving the rotors at the same speed and in opposite directions such that the gear teeth on the internal rotors slide along the wall of the rotor cavity from the inlet toward the outlet side of the pump.

3. A gear pump adapted to receive a, molten organic thermoplastic under pressure and to discharge it as a continuous sheet, comprising: a stationary case having a cavity therein for mating rotors, said cavity being in the form of two longitudinally intersecting right cylinders whose axes are parallel and spaced apart in a vertical plane a distance equal to the pitch diameter of the rotors operative therein; fiat end plates closing said rotor cavity and having bearings therein for a rotor centered in each of said cylinders constituting said cavity; two rotary members extending between the said bearings in the end plates, through said cavity, each comprising a rotor shaft on which is mounted and secured a plurality of identical multiple tooth gear Wheels filling the shaft between the end plates with no intervening space along the shaft, each gear wheel having an outside diameter substantially identical with the di- 7 meter of said cylinders ,to provide a sliding fit therein and a pitch diameter equal to the distance between'the said parallel axes of the cylindrical portions of the rotor cavity, each tooth on each wheel being ofiset from those on adjacent wheels on the same shaft, by from 0.15 to 0.4 times the circular pitch of the teeth; each two adjacent teeth on-each wheel having a fiute therebetween: each tooth, in elevation, exhibiting a smoothly rounded semicircular crown with no significant undercut and each interdental flute being smoothly semicircular of the same radius as the crown to provide an exact fit for its mating tooth; the teeth on one rotor being positioned as the mirror imagecomplements of the flutes on the other rotor to provide perfect mating between corresponding wheels on the two shafts; an inlet port through and centered in one wall of the case facing the parting line between the rotors; an internal distributor groove on the inlet side of the pump parallel to and facing the nip between the rotors and extending throughout their length; a pressure-actuated relief valve in a port extending from the distributor groove to the outside of the pump; an orifice extending between the end plates in communication with a horizontal passageway having a uniform small vertical dimension greater than that of the orifice and having the same length as the working face of the rotors, said passageway being in communication with the rotor cavity in the horizontal plane through the mating line between the rotors and on the opposite of the pump.

- on the internal rotors slide along the-wall of the rotor cavity from the inlet toward the outlet side FREDERICK E. DULMAGE. GRANT W. CHENEY.

REFERENCES CITED The following references are of record inthe file of this patent:

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